The invention relates to a transversely scanning magnetic-tape recording/playback apparatus, and more particularily to an apparatus of that type especially useful for recording/reproducing wide-band signals such as video signals on/from a magnetic tape having a plastic tape base with a coated side on which a magnetizable layer has been deposited and a back side (hereinafter referred to simply as a transversely scanning magnetic tape recorder). Such recorders typically include a magnetic head arrangement on a head support rotatable about an axis parallel to the direction of tape movement. At least two magnetic heads are regularly spaced along the support circumference, so as to move along circular paths of equal diameter. A concave tape guide member with concavely curved tape guide portions engages the magnetic tape on each opposite side of the head support for guiding the magnetic tape in a locally transversely-curved shape past the circular paths of the magnetic heads. A first cylindrical tape guide upstream from the concave tape guide member, and a second cylindrical tape guide downstream from the concave tape guide member, guide the tape in a transversely straight shape the cross-section of the magnetic tape during its movement from upstream to the cylindrical downstream tape guide changing from flat to curved and then from curved to flat.
An apparatus of this type is described in the Netherlands Patent Application No. 75 00730 to which U.S. Pat. No. 4,012,787 corresponds, whose disclosure is hereby incorporated by reference. In this known apparatus the magnetic tape is guided by convex tape guide portions, in addition to the concave tape guide member. The convex surfaces are opposite but slightly offset axially from the concave surfaces. In other known recorders of this general type the tape is guided solely by the concave tape guide means, and a partial vacuum in a chamber between two concave tape guides draws the tape against the curved guide portions. The transition, in either of these recorder types, between the cylindrical and concave guides causes a rather substantial change of the shape of the magnetic tape. In a practical example the magnetic tape has a width of 1 inch and the head wheel is dimensioned so that the magnetic heads cover a path with a diameter which is also approximately 1 inch. This means that the magnetic tape at the location of the path of the magnetic heads covers approx. 1/3 of the circular path. The thickness of the magnetic tape for this example may be 20 to 30 microns (0.0008 to 0.0012 inches). Such considerable changes in the shape of a tape cannot readily be analyzed in order to calculate the stresses causes in the tape.
The transition from a straight to a curved shape gives rise to changes in the membrane stresses in the magnetic tape. In this respect membrane stress is to be understood to mean a material stress which is the result of forces acting in the plane of the magnetic tape. Experiments have revealed that changes in the membrane stress during the deformation of the magnetic tape may result in the local occurrence of kinking and folding and in irregular contact pressures between the magnetic tape and the magnetic head at the locations where these contact the magnetic tape.
The distance between the straight tape guides, the pre-tension which is exerted on the magnetic tape in the direction of transport, the type and the dimensions in thickness and transverse direction of the magnetic tape and the diameter of the concave guide means all influence the extent to which these effects occur. The distance between the straight tape guides also affects the minimum dimensions of the entire apparatus. The pre-tension in the magnetic tape influences the contact pressure between a magnetic tape and the magnetic heads and thus the wear of the magnetic tape and the magnetic heads, and it moreover governs the thickness of the magnetic tape to be used. The thickness of the magnetic tape influences the maximum playing time that can be achieved with a supply reel of a given size. The width of the magnetic tape determines the dimensions of the head disc and the height of the supply and the take-up reel, and also influences the minimum distance between the straight tape guides which will not give rise to kinking and folding of the magnetic tape, and thus the dimensions of the apparatus.
It is evident that in order to minimize the dimensions of a recorder for which the most important of the aforementioned factors have been predetermined, it will be attempted to arrange the straight tape guides at a minimal distance from each other. This is of particular importance for tape cassette recorders, for example as disclosed in the previously cited U.S. Pat. No. 4,012,787. When such a cassette is used the upstream and downstream straight magnetic-tape guides may consist of rollers which are disposed in the magnetic tape cassette. The distance between these rollers determines the dimension of the magnetic tape cassette, or has at least great influence on the dimensions.
Therefore, it is generally attempted to minimize the distance between the upstream and the downstream tape guides. The occurrence of kinks or folds in the magnetic tape is to be avoided at any cost, because these may give rise to permanent deformations in the magnetic tape and, furthermore, to dynamic effects which may affect the read-out of a signal which is recorded on the magnetic tape. It is very important to minimize the average contact pressure between the magnetic tape and the recording heads so that minimal wear of the magnetic heads and the magnetic tape will result. In this respect it is not only of importance that the membrane stress in the cross-section at the location of the paths of the magnetic heads has a low average, but also that as far as possible the membrane stress is constant throughout the cross-section. This aspect will be explained in more detail with reference to the drawing.
Regarding contact pressure between the magnetic tape and the magnetic heads, two effects must be clearly distinguished from each other. First of all the uniformity of the contact pressure may be adversely affected by the variation in the membrane stress owing to the change in shape of the magnetic tape from straight to arcuate. Secondly, an irregularity in the contact pressure occurs because the magnetic tape in the middle of a cross-section has a higher resistance to depression, that is, a greater local stiffness, than at the edges. (The "edge effect" known from mechanics).
Experiments have revealed that the minimum distance between the upstream and downstream tape guides for which no folds or kinks in the magnetic tape occur, in other words the optimal geometry of the tape guide arrangement, depends on several factors.
1. There is found to be a substantial dependence on the correct position of the upstream and downstream tape guides relative to the concave tape guide.
2. The minimum distance between the upstream and the downstream tape guide is substantially inversely proportional to:
(a) the number of magnetic heads on the rotary head disc (because the number of magnetic heads for a specific given scanning speed of the magnetic heads along the magnetic tape determines the diameter of the head), PA1 (b) the average membrane stress in the magnetic tape, and PA1 (c) the square of the thickness of the magnetic tape.